1. Field of Invention
This invention relates generally to a nonlinear voltage variable resistor which is produced by molding and sintering metallic oxide mixtures, and, more particularly, to a method of manufacturing a high voltage metal oxide varistor.
2. Description of the Prior Art
Metal oxide varistors, consisting of a ceramic material, sintered at a high temperature, containing primarily one electrically conductive metallic oxide such as ZnO, TiO.sub.2, SnO.sub.2, or SrO.sub.2, with small amounts of other selected metal oxides or fluorides, are well known to the art. For example, U.S. Pat. No. 3,953,373, issued Apr. 27, 1976 to Matsuura et al, describes various compositions of metal oxide varistors in which the major conductive component is zinc oxide. U.S. Pat. No. 3,953,375, issued Apr. 27, 1976 to Nagano et al describes various compositions of metal oxide varistors in which the major conductive component is titanium oxide, and U.S. Pat. No. 3,899,451, issued Aug. 12, 1975 to Ichinose et al, describes various compositions of metallic oxide varistors in which the main conductive components are select mixtures of Zn.sub.O with TiO.sub.2, SnO.sub.2, or ZrO.sub.2. Additives which may be used in these metal oxide varistors include the oxides or fluorides of bismuth, cobalt, manganese, barium, boron, berylium, magnesium, calcium, strontium, titanium, antimony, germanium, chromium, nickel, lithium, indium, cerium, aluminum, tin, molydenum, vandium, tantalum and iron.
The manufacturing process for all such metal oxide varistors is similar. Accurately weighed quantities of metal oxides and additives, having predetermined composition ratio, are powdered and mixed together, generally by a ball mill. The mixture may be preliminarily calcined at a relatively low temperature in the range of 400.degree. C to 1000.degree. C and again pulverized in a ball mill. The powder thus obtained is mixed with a suitable binder such as water, polyvinyl alcohol, etc. and shaped under a pressure of about 50 to 1000 kg/cm.sup.2, into a disc or block having very smooth, planar, parallel top and bottom surfaces. These blocks are then sintered at a high temperature, in the range of 1000.degree. C to 1450.degree. C, for about 1 to 20 hours, then furnace-cooled to room temperature. The sintered blocks are provided at their respective top and bottom surfaces with ohmic electrodes applied by a suitable method such as silver painting, vacuum evaporation, or flame spraying of metals such as Al, Zn, Sn, etc. The top and bottom surfaces of the block may be lapped before the electrodes are applied thereon to assure a uniform thickness of the block.
Generally, the unfired varistor blocks are formed by uniaxially cold pressing the selected mixture of metal oxide powders and additives into a disc or block. In such a molding process, the thickness of the block thus produced is quite restricted, generally not exceeding the diameter of the block. Since the voltage rating of a metal oxide varistor is directly dependent on its thickness for any given formulation, when these varistor blocks are used in high voltage devices, such as lightning arresters for transmission or distribution power lines, it has heretofore been common practice to electrically connect a plurality of these varistor blocks in series via the two opposite metalized contacts of each varistor block or element. For example, in the lightning arrester disclosed in U.S. Pat. No. 3,959,543, issued May 25, 1976 to Howard F. Ellis, a plurality of vertically stacked metal oxide resistors are electrically connected in series by the fusing of silver paste layers previously applied to both faces of each metal oxide varistor disc.